1. Field of the Invention
The invention relates to antennas, and particularly to a planar antenna.
2. Description of Related Art
Wireless communication devices, such as mobile phones, wireless cards, and access points, wirelessly radiate signals via electromagnetic waves. Thus, remote wireless communication devices can receive the signals without the need for cables.
In a wireless communication device, the antenna is a key element for radiating and receiving radio frequency signals. Characteristics of the antenna, such as radiation efficiency, orientation, frequency band, and impedance matching, have a significant influence on performance of the wireless communication device. Nowadays, there are two kinds of antennas, built-in antennas and external antennas. Compared to the external antenna, the size of the built-in antenna is smaller, and the body of the built-in antenna is protected and not easily damaged. Thus, the built-in antenna is commonly employed in wireless communication devices. Common built-in antennas include low temperature co-fired ceramic (LTCC) antennas and printed antennas. The LTCC antenna has good performance at high frequencies and at high temperatures, but is expensive. A common type of printed antenna is the planar inverted-F antenna. Compared to LTCC antennas, planar inverted-F antennas are small, light, thin, and inexpensive. Accordingly, planar inverted-F antennas are mostly used in wireless communication devices.
In general, the planar inverted-F antenna is a printed circuit disposed on a substrate for radiating and receiving radio frequency signals. FIG. 1 is a schematic plan view of a conventional planar inverted-F antenna. The planar inverted-F antenna disposed on a substrate 10 includes a metallic ground plane 20, a radiating part 30, an open-short transforming part 40, and a feeding part 50. The metallic ground plane 20 is laid on the substrate 10, and includes an opening 60. The radiating part 30 includes an open end 31 and a first connecting end 33. The open end 31 terminates the radiating part 30.
The open-short transforming part 40 is connected between the radiating part 30 and the metallic ground plane 20, and includes a second connecting end 41 and a third connecting end 44. The third connecting end 44 is connected to the metallic ground plane 20. The second connecting end 41 is connected to the first connecting end 33 at a joint portion 70. The feeding part 50 is connected to the joint portion 70, for feeding signals. The feeding part 50 is connected to a matching circuit (not shown) through the opening 60.
In recent years, more attention has been paid on development of small-sized and low-profile wireless communication devices. Antennas, as key elements of wireless communication devices, have to be miniaturized accordingly. Although, the above-described planar inverted-F antenna is smaller than an external antenna, it is still too large for newer smaller wireless communication devices, and the profile of the above-described planar inverted-F antenna cannot be further reduced. Therefore, what is needed is another planar antenna with a miniaturized compact profile and better performance.